Derivatives and Hedging Fair Value of Derivative Contracts (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Hedged Liability, Fair Value Hedge	$ 0.0	$ 247.6
Hedged Asset, Fair Value Hedge	1,390.7	1,587.1
Derivative Asset, Fair Value, Gross Asset	38.4	3.3	$ 1.8
Derivative Liability, Fair Value, Gross Liability	96.3	86.1	55.5
Prepayable Financial Asset Closed Portfolio, Last-of-Layer, Amortized Cost	1,400.0	1,900.0
Hedged Asset, Fair Value Hedge, Last-of-Layer, Cumulative Increase (Decrease)	15.7	0.6
Hedged Asset, Fair Value Hedge, Last-of-Layer, Amount	880.0
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	13.8	3.3	1.8
Derivative Liability, Fair Value, Gross Liability	54.5	86.1	55.5
Forward purchase contracts
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	7.9	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	17.7	0.0	0.0
Forward sales contracts
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	15.5	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	17.6	0.0	0.0
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	1,382.8	1,689.9	863.0
Derivative Asset, Fair Value, Gross Asset	13.8	3.3	1.8
Derivative Liability, Fair Value, Gross Liability	54.5	86.1	55.5
Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	1,382.8	1,689.9	863.0
Derivative Asset, Fair Value, Gross Asset	13.8	3.3	1.8
Derivative Liability, Fair Value, Gross Liability	54.5	86.1	55.5
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	479,383.1	122.7	9.6
Derivative Asset, Fair Value, Gross Asset	35.2	0.9	0.1
Derivative Liability, Fair Value, Gross Liability	38.8	1.4	0.1
Not Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	4.1	3.5	2.9
Derivative Asset, Fair Value, Gross Asset	0.1	0.2	0.1
Derivative Liability, Fair Value, Gross Liability	0.1	0.2	0.1
Not Designated as Hedging Instrument [Member] \| Foreign Exchange Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	180.1	119.2	6.7
Derivative Asset, Fair Value, Gross Asset	0.2	0.7	0.0
Derivative Liability, Fair Value, Gross Liability	1.4	1.2	0.0
Not Designated as Hedging Instrument [Member] \| Forward purchase contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	11,713.5	0.0	0.0
Derivative Asset, Fair Value, Gross Asset	8.1	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	18.0	0.0	0.0
Not Designated as Hedging Instrument [Member] \| Forward sales contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	17,358.5	0.0	0.0
Derivative Asset, Fair Value, Gross Asset	15.8	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	17.7	0.0	0.0
Not Designated as Hedging Instrument [Member] \| Futures purchase contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	218,053.7	0.0	0.0
Derivative Asset, Fair Value, Gross Asset	0.0	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0	0.0
Not Designated as Hedging Instrument [Member] \| Futures sales contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	229,040.0	0.0	0.0
Derivative Asset, Fair Value, Gross Asset	0.0	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0	0.0
Not Designated as Hedging Instrument [Member] \| Interest Rate Lock Commitments
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	3,033.2	0.0	0.0
Derivative Asset, Fair Value, Gross Asset	11.0	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	$ 1.6	$ 0.0	$ 0.0

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624163-113959

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativesFairValueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of asset hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959

+ Details

Name:	us-gaap_HedgedAssetFairValueHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of asset hedged in last of layer, expected to be remaining at end of hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999718-113959

+ Details

Name:	us-gaap_HedgedAssetFairValueHedgeLastOfLayerAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, in last-of-layer hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999718-113959

+ Details

Name:	us-gaap_HedgedAssetFairValueHedgeLastOfLayerCumulativeIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of liability hedged in fair value hedging relationship.

+ References

+ Details

Name:	us-gaap_HedgedLiabilityFairValueHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amortized cost of closed portfolio of prepayable financial asset, used to designate hedging relationship in which hedged item is last layer expected to be remaining at end of hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999718-113959

+ Details

Name:	us-gaap_PrepayableFinancialAssetClosedPortfolioLastOfLayerAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wal_ForwardPurchaseContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wal_ForwardSalesContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wal_FuturesPurchaseContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wal_FuturesSalesContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: